Mri artifacts

my presentation about mri artifacts

1. MRI ARTIFACTS a.Shajitha B sc radiology Madras medical college

2. What is an artifact ?? An image artifact is a structure not normally present. But visible as a result of a limitation or malfunction in the hardware or software in the mri device. Some affect the quality of the MRI exam while others do not affect the diagnostic quality but may be confused with pathology. The knowledge of MRI artifacts and noise producing factors is important for continuing maintenance of image quality .

3. Sources of artifacts Hardware issues Software problems Physiological phenomena Physical limitations

4. Types of artifacts Chemical shift Aliasing Black boundary Gibbs or truncation Zipper artifact Motion artifacts Entry slice phenomenon Field inhomogenity Slice overlapping

5. Magic angle Moire fringes RF overflow Susceptibility Eddy current artifacts Center point artifact

6. Chemical shift artifact In the frequency direction, the MR scanner uses the frequency of the signal to indicate spatial resolution Since water in organs and muscles resonate at different frequency than fat. The MR scanner mistakes the frequency as a spatial difference. Commonly noticed in vertebral end plates, abdomen, and orbits where fat and other tissues form borders.

7. Appearance Spine -one end plate to appear thicker than the opposite one. Abdomen &orbits -Black border at fat water interfaces and bright border at opposite border. Solution fat suppression technique Use wide receiver bandwidth

8. Aliasing artifact or wraparound It occurs when the dimensions of the body part being imaged exceed the FoV. The part beyond the FoV is projected to the other side of the image. It is caused by under sampling in the phase encoded direction. Choosing an FoV that is smaller than the area imaged leads to wraparound or aliasing artifacts.

9. Appearance signals from outside the FoV seen in the imaging volume. Solution Larger FoV and oversampling Use saturation bands outside the FoV.

10. Black Boundary artifact Artificially created black line located at water fat interfaces. This results in sharp delineation of the muscle fat boundary that is sometimes visually appealing but not an anatomical structure. Occurs at TE when the fat and water spins located in the same pixel are out of phase, cancelling each others signal. Particularly noticeable in GRE sequences. Both in frequency and phase directions.

11. Remedy Use phase TE’s Fat suppression technique Increase matrix size and bandwidth.

12. Gibbs or truncation artifact Occurs at high contrast boundaries. Due to truncation(omission) of sampled signals. Commonly seen at the low signal intensity spinal cord with high signal intensity CSF on T2WI of the spine. As the signal is sampled, some data is necessarily omitted in k-space, causing the signal intensity of a given pixel to vary from its ideal signal intensity. Appearance Bright and dark lines. Solution Increase matrix size.

13. Motion artifacts Voluntary/involuntary patient motion. Blurness in the image. Caused by Arterial pulsations CSF pulsations Swallowing Breathing Peristalsis Physical movement

14. Solutions Patient education Sedation Motion correction sequences e.g. HASTE Navigator echo Respiratory gating Cardiac triggering

15. Entry slice phenomenon When unsaturated spins in blood first enter in to a slice or slices. Characterized by bright signal in a blood vessel at the first slice. Confused with thrombosis. Gradient echo flow techniques to differentiate. Solution Use spatial saturation bands before the first and after the last.

16. Field inhomogenity Contribute to signal non uniformity. May arise from RF coil construction or standing wave effects. When the RF is shorter than the dimensions of the anatomic structures examined standing waves may result. Leads to inhomogeneous fat suppression.

17. Solution Shimming –allow precise control of overall homogeneity of RF field. Use STIR for Fat Sat. Coil- use volume coils, allow space between body and coil. Dielectric –use phased array coils.

18. Slice overlapping Loss of signal due to multi slice, multi angle acquisition or imperfect slice profile. Mechanism : Spin saturation. If slices at different angles cross, then spins that have previously excited, could be excited again. Mostly seen in spine imaging e.g. L4-L5 or L5-S1.

19. Appearance Band of signal loss crossing horizontally in the image, usually posteriorly. Solution Continuous imaging Increase slice gap.

20. Magic angle artifact Seen most frequently in tendons and ligaments that are oriented at a 55 degree to main magnetic field. Normally dipolar interaction between water molecules in ligaments are strong. Which implies that T2 relaxation is very fast leading to signal loss. Dipolar interaction go to zero. Solution Lengthen TE Use T1 weighted sequences since T1 relaxation is unaffected by this.

21. Moire fringes An interference pattern most commonly when doing gradient echo images. One cause is aliasing of one side of the body to the other results in superimposition. Can also be caused by receiver picking up a stimulated echo. Similar to the effect of looking through two window screens.

22. Solution Improve shimming

23. RF overflow artifact Non uniform, washed out appearance in an image. Occurs when the signal received from the patient is too intense to be digitized by analog to digital converter. Auto prescanning usually adjusts the receiver gain. Post processing methods also existing but may be time consuming.

24. Magnetic suscepbility artifact Distortion in the MR image especially seen in while imaging with metallic orthopedic hardware or dental work. Magnetic field inhomogenetics introduced by the metallic object in to homogeneous magnetic field. Greater at high magnetic field strength. Worst with long TE and gradient echo sequences.

25. Appearance Bright and dark areas Solution Larger receiver bandwidth Gradient echo and echo planar sequences should be avoided. The use of echo spin particularly fast spin echo sequences should be considered.

26. Eddy current artifact Varying magnetic field induces electric current which distort gradient waveforms. When diffusion gradient applied, change in magnetic field creates electric current . Such current creates smaller magnetic field that Bo. Modern gradient coils equipped with active shielding to avoid these effects of electric conduction.

27. Solution Shielding gradients. A distorted gradient waveform is used

28. Central Point artifact A focal dot of increased or decreased signal in the center of the image. Caused by constant offset of the DC voltage in the amplifiers . Solution Requires calibration. Maintain a constant temperature.

29. Zipper artifact Most of them related to hardware or software problems. RF from some radio transmitters will cause zipper that are oriented perpendicular to frequency direction. Occur in either frequency or phase encoding directions . Solutions MR scanner room is shut down. Remove all electric devices from patient.

30. Spike artifact Caused by one bad data point k-space. Shows one data point in k-space, which is out of ordinary. Diagonal lines in the image . Solution Repeat the scan

31. Annefact artifact Due to anatomy within the active volume of the coil, but outside the Fov. Signals from such region give rise to ghost imaging in the phase encoding direction. Most commonly in sagittal spine imaging especially in T and L spines. Always turn off the coils which is not in use.